I wonder whether my phone provider can legally upgrade my phone's operating system without my consent.
I am most interested in the United States, especially California and Massachusetts.
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No, they can't legally update it without your consent.
(This includes consent in the form of a document you agreed to such as usage documents by AT&T, as well as the stuff Google has you accept before you use their property (like the Android OS, for example).)
You entered into a license with the phone OS provider when you started using the phone. They can do whatever it says.